Waas 36?
On a Garmin 60c

I saw a very solid outline of satellite 36 this morning. It never got a lock after about 15 minutes but the outline on the satellite page was very strong. I have had a lock on 35 and 47 before but have never even seen a blip for other signals in the WAAS range.

I saw a very solid outline of satellite 36 this morning. It never got a lock after about 15 minutes but the outline on the satellite page was very strong. I have had a lock on 35 and 47 before but have never even seen a blip for other signals in the WAAS range.

Any ideas of what this is?

SV 36 is not a WAAS bird, but one of many in the GPS constellation. Here's a summary of GPS 2A-24 (SV 36).

Interesting. That appears to be the case. Odd that I've not seen 36 before. Odd also that I received the following reply from a Garmin tech a month or so ago about the modified WAAS search sequence on a 60c (was changed in beta 4.02 and is also in beta 4.04):

Bob,

The only difference will be in the search order for WAAS. Satellites with known almanac will be searched for first. If they are not found on the first pass, then the other satellites will be tried in numerical order, but the satellites with known almanac will be interspersed every other attempt.
So a search order that used to be something like:

Aren't the WAAS birds in geostationary orbits over the equator? But PRN36 is somehow positioned north of Seattle? Maybe it takes some time after launch to move the satellite properly into synchronous orbit?

Assuming the 60c satellite page was providing a correct indication...that would appear to be correct. However, since it did not have a solid lock, and it was displayed as due north, perhaps that's the default location until a lock is obtained and the almanac data downloaded....?

Yep, that's what it does with 35 and 47 before the GPS acquires a correct almanac. I'd guess that the FAA is probably just testing them, it might not even be transmitting actual WAAS corrections. They're not scheduled to go active until near the end of the year according to the FAA site.

...........
The SV (satellite or space vehicle) number is not what you see on a GPS. It's the PRN which in this case for SV36 is 06 Navstar GPS Constellation Status

I'm not sure what would be using PRN 36.....

I thought that Garmins report the satellite numbers differently to Magellans(which report the PRN).
So I assumed that the only other alternative would have been SVN??
And if THIS info is to be believed the SVN and PRN are sometimes the same!

This site indicates that Garmins number the satellites according to Garmins random protocol!!

Assuming the 60c satellite page was providing a correct indication...that would appear to be correct. However, since it did not have a solid lock, and it was displayed as due north, perhaps that's the default location until a lock is obtained and the almanac data downloaded....?

The second part is correct. ("Locks", solid or otherwise, have nothing to do with it.)

If your Garmin GPSr showed number "36", this indicates PRN123.

The identifer PRN123 is not allocated for any current or proposed GPS-related service, as far as I can tell. The fact that your GPSr did not find an almanac entry for such an SV, confirms that it is not an operational signal.

The only explanation is that you saw EITHER a data error, OR test signals - from a satellite or a ground station.

GPS units ignore anything that isn't in the almanac, and we can safely do so too.

The GPS definitely displayed 36 which is in the range that Garmin considers a WAAS satellite. However, since that one experience I have not seen any indication of 36 which suggests it was either a malfuntion of the unit, or a test signal of some sort.

There was one launched in September and one in October. One of them is 123, but I don't know which. I could find out if you really care. They are two completely different satellites that have the WAAS payload on them, so they might have completely different test schedules.

One (on ANIK F1R) is supposed to become operational in September and the other (on PanamSat Galaxy 15) in October. I haven't yet been able to find what PRNs they have been assigned.

What's more, I have had a very hard time figuring out exactly which orbital slot the PanamSat Galaxy 15 satellite actually got. It was originally supposed to be 125 W, then 133 W, and recently I found a reference that put it at 144 W last November. Maybe it is still moving to its final position? Who knows....

I have also been looking at the various websites I know of. My question: Will these WAAS SV's replace the existing ones? If not, where will they go? I mean, the US has pretty good coverage already with WAAS..... and you only need one..... FAA website lacking lots of info on plans. And the REAL civilian GPS operator (the US Coast Guard Navigation Center) barely mentions it. Just curious.

This post has been edited by Klemmer & TeddyBearMama: 09 January 2006 - 01:29 PM

Apparently the leases on the existing WAAS satellites are going to run out pretty soon, so in that sense, they will indeed be "replacing" the existing WAAS satellites. But they will not be in the same positions.

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If not, where will they go?

The existing WAAS satellites are not placed exactly where you might like them to be; in particular, Alaska and the Northwest don't get very good coverage. As far as I can tell, ANIK F1R is at 107 W and Galaxy 15 is at 133 W. Those positions will provide greatly improved coverage for the US mainland and Alaska.

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and you only need one

If you're going to be using the system to land planes, it better be redundant! That is one of the major problems with the current WAAS system; if the POR satellite goes down, Alaska is completely out of luck.

The existing WAAS satellites are not placed exactly where you might like them to be; in particular, Alaska and the Northwest don't get very good coverage. As far as I can tell, ANIK F1R is at 107 W and Galaxy 15 is at 133 W. Those positions will provide greatly improved coverage for the US mainland and Alaska.

Oh, yeah! That's how they were going to do dual coverage. I do remember hearing at one point about moving the old Inmarsats as well, but that might have been before project delays. If the lease is up soon, they might not want to any more. Especially if they want to migrate to L5 for aviation.

I got a solid indication of 36 again this morning on my 60c, but could not get a lock. This presents an intriguing situation, as it essentially prevents the 60c from getting WAAS correction. The new algorithm in the 4.0x betas will only search for one WAAS signal if there are 11 non-WAAS signals. If the one WAAS signal happens to be 36, it will not search for any other WAAS satellite such as 35 or 47. Since 36 is apparently non-operational, the 60c will never lock but will never drop it and search for another.

IMHO the WAAS acquisition algorithm has been flawed on the 60/76 C/CS firmware for a while now. Why does the unit cycle through numerous PRNs that do not currently exist or that it could not possibly see? After all the unit does know it's location so why does it look for geos on the other side of the globe?

If the one WAAS signal happens to be 36, it will not search for any other WAAS satellite such as 35 or 47. Since 36 is apparently non-operational, the 60c will never lock but will never drop it and search for another.

If that is true, it's A Bad Thing...

...But it may not be.

Your 60C should ignore that station - but it won't "know" to do that until it has loaded - error free - the entire WAAS almanac.

This, as we know, can take some time - but I'd expect to see the unit resume searching after an interval.

Here is what Garmin told me about the search algorithm on the 60c:
Bob,

There are a couple reasons why we search for satellites that we know do not exist at the current time:

1. Our goal is to have the unit work out of the box without any need to update at any time. More and more of our customers now will never look for updates online and will never hook the unit up to a PC - we try to design our software with this in mind. This search algorithm is flexible enough to account for new satellites coming up, old satellites going dead or changing positions, and just about everything else the WAAS/EGNOS folks could do to try to confuse our units.

2. In a perfect world, all SBAS(WAAS/EGNOS/MSAS) satellites would transmit the location of every possible SBAS satellite out there. This would eliminate most of the need and complexity from the search algorithm.
However, this is not the case. WAAS satellites only transmit the locations of other WAAS satellites. The EGNOS system only transmits the location of other EGNOS satellites. In order to account for multiple and possibly overlapping service providers that don't transmit anything about each other, we need to scan the unknown satellite PRNs if none of the known satellites are found. (Just in case I lost you with the terminology: SBAS(Satellite Based Augmentation System) is the overall system. WAAS(US), EGNOS(Europe),
MSAS(Japan) are the separate service providers that make up the entire set of SBAS satellites)

In the normal case you are right, we don't need to search for these satellites. But we have to design for the exceptional cases and that is the main reason for the scan of the unknown PRNs. I think my managers will agree with all of the above, but I will forward your concerns along to them.

I logged data off of 123 last night. It was broadcasting from about 4:40pm to 8:54am MST. And I see it's started again just a little bit ago. The data was nothing but message 42 filled with alternating 1's and 0's, essentially just junk.

The WAAS almanac is only broadcast by the WAAS satellites. So if you haven't tracked one for a while, a GPSr might think it's saved one is too old and do the full search. It also will usually give up on the ones it knows about if they're blocked. If it can't find them after some period of time, it has to assume they're gone or will be continue to be blocked indefinitely. At that point it has to search for unknown satellites, possibly from a different system (like EGNOS) that it wouldn't know about from the WAAS almanac.

That's assuming Garmin saves the WAAS almanac at all.

Edit: Oops. Bob got in there while I was typing. So Garmin might be giving up on the known sats a little coservatively. Either way, it should be timing out on 123 when it doesn't get any usable data for a while. I'm going to hook up my Legend to see what it does.

I'll see what my Magellan does this afternoon for WAAS. I think I can "dump" the almanac by turning off WAAS (secret menu on my Meridian), locking it up, then cycle power & turn on WAAS again. Will advise what Magellan thinks of SV123, if anything.

It looks like the signal is on most of the time: I'm getting strong indicators, although no lock. Ironically, I can't lose the signal on my 60c (which prevents it from searching for other WAAS signals), and my Map76 won't ever find it since its search algorithm only looks at 35 and 47.

...
Ironically, I can't lose the signal on my 60c (which prevents it from searching for other WAAS signals),...

That's certainly something that should be considered a bug that Garmin needs to address. It should timeout on a geo that doesn't yield any usable messages. Does it even start searching again if you block the antenna in that direction?

I can't get my Legend to give up on 35 and 47, and I don't really want to do a factory reset to see what it does with 36.

I agree that is a bug that Garmin should address. I have not tried blocking the 36 signal, but my guess is that would work and the unit would eventually give up searching for 36, which would allow it to search for other satellites. Of course, another option is to turn the unit off and back on when I have visibility to 35 or 47. As for the Legend, it sounds like it searches like theMap76: it once found 35 and 47, and therefore has concluded those are the only WAAS signals. It was nice when that was true...

I picked up 36 today while out caching at N53 W117. As others have experienced my GPS (a 76C) showed a hollow signal strength bar and no "D"s were displayed because of the test signal. What was encouraging (for me anyway) was that while my GPSr couldn't acquire 35 or 47 it got a good signal from 36 (over 4 on the scale bars).

This was in a small clearing in a forest and bodes well for this fall when the new sats goes into service.

Yes, not only is it more centrally located, it's transmitting a stronger signal than the Inmarsats are. They are also adding reference stations in Canada and Mexico to improve the ionosphere coverage. It should be a nice improvement altogether.

I picked up 36 today while out caching at N53 W117. As others have experienced my GPS (a 76C) showed a hollow signal strength bar and no "D"s were displayed because of the test signal. What was encouraging (for me anyway) was that while my GPSr couldn't acquire 35 or 47 it got a good signal from 36 (over 4 on the scale bars).

This was in a small clearing in a forest and bodes well for this fall when the new sats goes into service.

Same with me today. I picked up sat 36 while moving in a car at N 53 W 113. No lock, but strong test signal tough. I 'appeared' to be sitting on the horizon of the Northern sky, but from reading previous post, it looks like this is normal until it gets a lock.

The test signal from 36 seems stronger that what I ever had with 35 and 47. Sounds good, turn it on!!!

I heard from a Garmin today today that even they were surprised by the new test signal. He said they're going to update some of their firmware to take it into account.

I wish they'd make selection of which WAAS sats to look for a user selectable menu item. At present in situations where reception of the WAAS corrections is intermittent the firmware wastes a lot of time trying the other PRNs when attempting to reacquire the sats.

Maybe this will become less of a problem with the increased signal strength of the new WAAS geos.

An update on WAAS 36: this morning it appears that I was receiving differential data from 36 on my 60c as the other satellites were displaying a "D", one-by-one. I did not have any other WAAS satellite.

In addition, satellite 36's location was displaying on the satellite page as being south-southeast of my location (Seattle, WA area), near the 45 degree circle. Previously it was displaying as due north on the horizon, which I inferred was the default location until it received accurate data.

An update on WAAS 36: this morning it appears that I was receiving differential data from 36 on my 60c as the other satellites were displaying a "D", one-by-one. I did not have any other WAAS satellite.

In addition, satellite 36's location was displaying on the satellite page as being south-southeast of my location (Seattle, WA area), near the 45 degree circle. Previously it was displaying as due north on the horizon, which I inferred was the default location until it received accurate data.

Confirmed. 123(36) is broadcasting in "test mode" (flagged as not usable for aviation purposes but available at your own risk).

Curiously, though, it's not listed in its own almanac. So you might see it use 36 for a while and then it will switch to another one like it's told (depending on your GPSr firmware). (edit: actually the almanac keeps changing)

Also of note is that 35 and 47 have been occasionally flagged as "bad health" the last couple of weeks. It looks like Inmarsat is moving AOR-W a little. Some of the time the health has been set to 7 (ranging off, corrections off, and integrity off) meaning it’s totally unusable. For the last day and a half, 35 has had a health of 1 (ranging off) meaning you can get corrections but it shouldn’t be used in the solution. You’ll probably see a hollow lock bar but get ‘D’s anyway.

An update on WAAS 36: this morning it appears that I was receiving differential data from 36 on my 60c as the other satellites were displaying a "D", one-by-one. I did not have any other WAAS satellite.

In addition, satellite 36's location was displaying on the satellite page as being south-southeast of my location (Seattle, WA area), near the 45 degree circle. Previously it was displaying as due north on the horizon, which I inferred was the default location until it received accurate data.

Confirmed. 123(36) is broadcasting in "test mode" (flagged as not usable for aviation purposes but available at your own risk).

Curiously, though, it's not listed in its own almanac. So you might see it use 36 for a while and then it will switch to another one like it's told (depending on your GPSr firmware). (edit: actually the almanac keeps changing)

Also of note is that 35 and 47 have been occasionally flagged as "bad health" the last couple of weeks. It looks like Inmarsat is moving AOR-W a little. Some of the time the health has been set to 7 (ranging off, corrections off, and integrity off) meaning it’s totally unusable. For the last day and a half, 35 has had a health of 1 (ranging off) meaning you can get corrections but it shouldn’t be used in the solution. You’ll probably see a hollow lock bar but get ‘D’s anyway.

This morning, I was in my car and I was getting correctional data for all my sats and an accuracy of +- 2 meters! Sat 35 and 47 were nowhere to be found on the sat list, but sat 36 was bouncing (trying to get a lock). So even though lock on sat 36 can't be aquired as regular GPS sat, it does transmit differential data. Here's the proof:

In addition, satellite 36's location was displaying on the satellite page as being south-southeast of my location (Seattle, WA area), near the 45 degree circle. Previously it was displaying as due north on the horizon, which I inferred was the default location until it received accurate data.

So which one is it? Both observers are from the NW but are seeing at different locations (SSE vs SSW)